I'm not sure what you are questioning about God's miracles.
Miracles are things that God does that are outside the normal physical laws. There have been various times in history when God has used major, public miracles to authenticate His message. Other times, such as today, we see fewer major miracles (or it may be that we just don't accept the miracles we see)--however, God still works miracles in order to accomplish His purposes. For example, I have been involved in three miraculous healings which were needed in order for this ministry to happen.
Satan is also able to simulate miracles, such as in Exodus chapter 7 when Pharaoh's magicians duplicate some of the plagues God sent. Today Satan's "miracles" are most often seen in occult practices.